How Hard-Headed Lizards Lost Their Legs

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The mysterious "worm lizard" has finally found its evolutionary
home; the legless animals are closely related to a group of
lizards named the lacertids, a new fossil intermediate indicates.

The worm lizards, also called amphisbaenians, look so similar to
primitive snakes that researchers weren't clear which group
they belonged to, the snake or lizard family.

"They look like, basically, worms, or a group of snakes that we
call blind snakes," said study researcher Robert Reisz, of the
University of Toronto, Mississagua, in Canada. "They burrow for a
living and eat grubs."

This new fossil, named Cryptolacerta hassiaca, has an
amphisbaenian-like reinforced head but retains
chunky, lacertid-like limbs. The fossil was discovered in the
so-called Eocene Messel site near Frankfurt, Germany, and
provides the first skeletal evidence that the two groups are most
likely related.

"These animals first evolved a shovel-like head, burrowed
headfirst with a shovel-like mechanism, then after that lost its
limbs," Reisz told LiveScience. "There are lots of things about
the head that tell us it was well on its way to be a
amphisbaenian."

The 47-million-year-old fossil is well preserved and represents
the only known example of its species. At the time of its death
it would have been considered a "living fossil," having survived
unchanged from its ancestors for millions of years. There are
other, older fossil examples of the amphisbaenians, but this is
the first example of the intermediate stage between the lacertids
and the amphisbaenians.

The fossil organism most likely lived in the leaf litter on the
forest floor. It used its reinforced head to dig in the dirt and
leaves, though it most likely didn't live completely underground.

Throughout evolution,
animals have lost their limbs several times, so it's not
unfathomable that these two species developed similar body plans
and burrowing strategies separately, the researchers say.

"We have two groups of animals — the blind snakes and these
limbless lizards — resembling each other to a great degree, but
they developed this body design independent of each other," Reisz
said. "There are lots of ways
you can lose your limbs, and this one gives us an example of
how it happens in these strange little subtropical lizards."

The study was published today (May 18) in the journal Nature.

You can follow LiveScience staff writer Jennifer Welsh on
Twitter @microbelover.
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